Method and apparatus for supplying gaseous nitrogen to a laser beam machine

ABSTRACT

Oxygen and nitrogen contained in air is separated by an air separator. The separated nitrogen gas is supplied to an optical path cover of a laser beam machine as a protective gas for bend mirrors. Further, the separated nitrogen gas or oxygen gas can be selectively supplied to a laser beam head as an assist gas. The purity of the separated nitrogen gas can be kept within a predetermined range (94 to 99.5%) on the basis of a difference in pressure between a first conduit for supplying nitrogen gas and a second conduit for supplying oxygen gas. It is preferable to provide a filter for removing dust from air supplied to the air separator and a second filter for removing oil mist from the separated nitrogen supplied to the optical path cover.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to a method and apparatus for supplyingnitrogen gas to an optical path system of a laser beam machine, and to alaser head provided for the same. Nitrogen gas is supplied to an opticalpath system to protect bend mirrors arranged in the optical path system,and further to a laser beam head as an assist gas.

2. Description of Background Information

As is well known, a laser beam machine is provided with a laser beamoscillator and a laser beam head. Further, a plurality of bend mirrorsare arranged between the laser beam oscillator and the laser beam head,to guide the laser beam generated by the laser beam oscillator to thelaser beam head.

The laser beam path from the laser beam oscillator to the laser beamhead is referred to as an optical path system, and is usuallypartitioned from outside air by a pipe member, for safety purposes andfor protection of the optical path system from dust.

Further, in known laser beam machines having a movable laser beam head,since the optical path length from the laser beam oscillator to thelaser beam head changes, both are connected by use of a bellows or atelescopic tube, etc.

In order to prevent outside air from entering the optical path system inthis type of system, dry air cleaned by an air drying unit is usuallysupplied into the optical path system to protect various opticalelements such as bend mirrors, lenses, etc.

Since the dry and clean air is supplied into the optical path system forprevention of outside air from entering the optical path system, dustlevels are kept extremely low, compared with the outside air. However,there exists a problem in that the optical elements deteriorate morerapidly due to exposure to and/or very small amounts of moisturecontained in the dry and clean air, when the laser beam machine has beenused for many hours.

In addition, an assist gas such as air, oxygen, nitrogen, argon, etc. isusually used in the laser beam machines. The appropriate assist gas isselected according to the sort of plate materials, laser beam machiningconditions, etc.

However, air, oxygen and nitrogen are generally chosen as the assistgases, because argon is costly (except the case where the material to beprocessed is titanium).

In the case where air is used as the assist gas, a compressed air can beobtained easily by use of a compressor. In the case of oxygen ornitrogen, however, an oxygen bomb or a nitrogen bomb must be prepared,so that the assist gas is not economical.

To overcome these problems, Japanese Published Unexamined (Kokai) PatentNo. 5-84590 (referred to as a prior art, hereinafter) discloses such aconstruction that oxygen and nitrogen in air are separated from eachother by an air separator so that the separated oxygen and nitrogen canbe used as the assist gas. In this method, since oxygen and nitrogen inair can be used after separation without using any gas bombs, the assistgas can be obtained at a relatively low cost.

In the above-mentioned method, however, since the pressure of the assistgas supplied to the laser beam head must be adjusted according to thelaser beam machining conditions, there exists another problem in thatthe purity of oxygen or nitrogen separated by the air separator varieswhenever the assist gas pressure is adjusted. This results in thepotential that a harmful influence will be produced upon the laser beamprocessing, if the purity of the assist gas varies out of a normallyacceptable range.

SUMMARY OF THE INVENTION

With these problems in mind, therefore, it is an object of the presentinvention to provide a method and apparatus for protecting the opticalpath system more reliably from deterioration (due to oxidation,moisture, etc.).

Another object of the present invention is to effectively use separatednitrogen as an assist gas and an optical path system protective gassimultaneously.

A further object of the present invention is to supply separatednitrogen gas as an assist gas within a constant purity range (94 to99.5%), even when the assist gas pressure is adjusted according to thelaser beam processing conditions.

To achieve the above-mentioned objects, the present invention provides amethod of supplying nitrogen gas into an optical path system of a laserbeam machine. The method includes: supplying compressed air into an airseparator for separating oxygen and nitrogen from the suppliedcompressed air; and introducing nitrogen-rich gas separated by the airseparator into an optical path cover of an optical path system of alaser beam machine, to maintain an inner pressure of the optical pathcover at a pressure higher than outside air pressure.

Further, it is preferable that the method comprises passing thenitrogen-rich gas separated by the air separator through a filter forremoving oil mist, before the nitrogen-rich gas is introduced into theoptical path cover of the optical path system of the laser beam machine.

The present invention further provides an apparatus for supplyingnitrogen gas into an optical path system of a laser beam machine. Theapparatus includes an air separator for separating oxygen and nitrogenfrom compressed air; and conduit means for introducing nitrogen-rich gasseparated by the air separator into an optical path cover of the opticalpath system of the laser beam machine.

Further, it is preferable that the apparatus also includes a firstfilter for removing dust from the compressed air, before the compressedair is supplied to the air separator; and a second filter for removingoil mist contained in the nitrogen-rich gas, before the nitrogen-richgas is introduced into the optical path cover of the optical path systemof the laser beam machine.

Further, the present invention provides a laser beam machine, comprisingan air separator for separating oxygen and nitrogen from compressed air;first conduit means for introducing nitrogen-rich gas separated by theair separator into an optical path cover of the laser beam machine as aprotective gas; and second conduit means for introducing thenitrogen-rich gas separated by the air separator into a laser beam headof the laser beam machine as an assist gas.

Still further, it is preferable that the laser beam machine includes afirst filter for removing dust from the compressed air, before thecompressed air is supplied to the air separator and a second filterdisposed in the first conduit means, for removing oil mist contained inthe nitrogen-rich gas, before the nitrogen-rich gas is introduced intothe optical path cover of the laser beam machine.

The second conduit means, preferably comprises a conduit connectedbetween the optical path cover and the laser beam head, for supplyingthe nitrogen-rich gas into the laser beam head as the assist gas.

Preferably, the second conduit means comprises a branch conduitconnected between the air separator and the laser beam head, forsupplying the nitrogen-rich gas into the laser beam head as the assistgas.

Still further, the second conduit means comprises a first conduitconnected between a first outlet port of the air separator, fordischarging the nitrogen-rich gas; and a second conduit connectedbetween a second outlet port of the air separator, for discharging theoxygen-rich gas.

It is preferable that the laser beam machine includes a firstchange-over valve disposed midway of the first conduit, for supplyingthe nitrogen-rich gas into the laser beam head when opened; and a secondchange-over valve disposed midway of the second conduit, for supplyingthe oxygen-rich gas into the laser beam head when opened.

It is further preferable that the laser beam machine includes a firstpressure control valve disposed midway of the first conduit, forcontrolling pressure in the first conduit; and a second pressure controlvalve disposed midway of the second conduit, for controlling pressure inthe second conduit.

Purity of the nitrogen-rich gas increases when the pressure in the firstconduit is increased by the first pressure control valve, but decreaseswhen the pressure therein is decreased thereby.

Alternatively, purity of the nitrogen-rich gas increases when thepressure in the first conduit is kept constant by the first pressurecontrol valve and when the pressure in the second conduit is decreasedby the second pressure control valve, but decreases when the pressure inthe first conduit is kept constant by the first pressure control valveand when the pressure in the second conduit is increased by the secondpressure control valve.

Purity of the nitrogen-rich gas can be kept roughly (substantially)constant when a difference in pressure between the first and secondconduits is kept roughly (substantially) constant.

Preferably, the laser beam machine further includes: a first controlvalve for remotely controlling the first pressure control valve; and asecond control valve for remotely controlling the second pressurecontrol valve.

Further, the laser beam machine preferably comprises a link mechanismfor controlling the first and second pressure control valvessimultaneously in linkage so that a difference in pressure between thefirst and second conduits can be kept substantially constant.

In the laser beam machine according to the present invention, since thenitrogen-rich gas containing an extremely small amount of oxygen andmoisture is supplied into the optical path system at a pressure higherthan the atmospheric pressure, it is possible to effectively preventfire accidents in the optical path system and to protect the opticalparts from deterioration due to oxidation and moisture, as compared withthe conventional dry air.

Further, in the laser beam machine according to the present invention,since the nitrogen-rich gas can be also supplied to the laser beam headas an assist gas, it is possible to effectively use the nitrogen-richgas separated by the air separator. Oxygen gas separated by the airseparator can be also used as an assist gas.

Since, the nitrogen-rich gas can be supplied to the laser beam head asan assist gas by keeping the purity of the nitrogen-rich gas or theoxygen-rich gas at a desired constant level or in a predetermined range(e.g., 94 to 99.5%) at all times, it is possible to prevent theoccurrence of dross during the laser beam processing.

Further, since the first filter removes dust from air supplied to theair separator and the second filter removes oil mist from the separatednitrogen supplied to the optical path cover, it is possible to improvethe lifetime of the optical parts of the optical path system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration showing a first embodiment of thelaser beam machine according to the present invention;

FIGS. 2A and 2B are illustrations for assistance in explaining the oilmist adhesion test method and the test results, respectively;

FIG. 3 is a schematic illustration showing a second embodiment of thelaser beam machine according to the present invention; and

FIG. 4 is a schematic illustration showing a third embodiment of thelaser beam machine according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the laser beam machine according to the present inventionwill be described hereinbelow with reference to the attached drawings.

In FIG. 1, the laser beam machine 1 is provided with a laser beamoscillator 3 and a laser beam head 7 having a condenser lens 5. Thelaser beam oscillator 3 is connected to the laser beam head 7 via anoptical path system 13 composed of a plurality of bend mirrors 9 and anoptical cover 11. The bend mirrors 9 guide a laser beam LB generated bythe laser beam oscillator 3 to the laser beam head 7. The optical cover11 is an appropriate pipe member, bellows, telescopic tube, etc.Further, the optical path system 13 is the same as with the case of theprior art one, so that any detailed description thereof is omittedherein.

Additionally, an air separator 15 is provided to separate oxygen andnitrogen from compressed air. The separated nitrogen-rich gas issupplied into the optical path cover 11 to protect the optical pathsystem 13. The air separator 15 is a module in which a great number ofhollow threads (e.g., formed of polyimide) are arranged in a lump withina vessel. The hollow polyimide thread membrane is more permeable tooxygen than to nitrogen.

Therefore, when compressed air is supplied into the air separator 15through an inlet port 15A and flows through the hollow threads, oxygen,moisture and oil mist of the compressed air are selectively passedthrough the membranes of the polyimide hollow threads. Thus, it ispossible to obtain a nitrogen-rich gas (purity: 94 to 99.5%) from afirst outlet port 15B. On the other hand, the permeated oxygen, moistureand oil mist are discharged through a second outlet port 15C asoxygen-rich gas. The above-mentioned nitrogen-rich gas (obtained byremoving oxygen, moisture and oil mist from air) is dry air (nitrogen)having a dew point of about -50 degrees under atmospheric pressure. Thecomparison with the ordinary refrigerator drier having a dew point ofabout -10 degrees under atmospheric pressure, it can be understood thatthe nitrogen-rich gas is extremely advantageous when used to protect theoptical path system 13.

To supply compressed air into the air separator 15, a pressure source17, such as a compressor, is provided. Further, a filter 19 is connectedbetween the pressure source 17 and the inlet port 15A of the airseparator 15, to remove dust and oil mist contained in the highpressurized air supplied by the pressure source 17.

A conduit (pipe) 23 is connected between the first outlet port 15B ofthe air separator 15 and a junction port 21 of the optical path cover11, to supply the nitrogen-rich gas separated from the compressed air bythe air separator 15 into the optical path cover 11 of the optical pathsystem 13. The second outlet port 15C of the air separator 15 is open tothe atmospheric pressure.

In the above-mentioned construction, after having been passed throughthe filter 19 for removal of dust and oil mist, the compressed air ofthe pressure source 17 is supplied to the air separator 15 through theinlet port 15A. The compressed air supplied into the air separator 15through the inlet port 15A is separated into oxygen (containingmoisture, residual oil mist (passed through the filter 19)) and nitrogenwhen it passes through the hollow thread membranes. As a result, it ispossible to obtain nitrogen-rich gas (purity: 94 to 99.5%) through thefirst outlet port 15B and the oxygen-rich gas passed through the secondoutlet port 15C of the air separator 15, respectively.

Since, as noted previously, the nitrogen-rich gas is dry air having adew point of about -50 degrees under atmospheric pressure, when thenitrogen-rich gas is supplied into the optical path cover 11 of theoptical path system 13 through the conduit 23, it is possible tomaintain the pressure within the optical path cover 11 higher than theatmospheric pressure.

In other words, since the optical cover 11 is kept at a pressure higherthan the atmospheric pressure and further filled with nitrogen-rich dryair containing extremely small contents of oxygen (nitrogen purity: 94to 99.5%), it is possible to protect the optical cover 11 from fireaccidents as well as to protect the optical parts (e.g., the bendmirrors 9) from deterioration (e.g., due to oxidation or moisture) moreeffectively.

In addition to introducing the nitrogen-rich gas into the optical pathcover 11 for protection of the optical path system 13, as describedabove, it is also possible to direct the dry air against the bendmirrors 9 to clean the surfaces of the bend mirrors 9. To check theeffectiveness of the direct jetting of the dry air against the bendmirrors 9 for surface cleaning, the following tests were made:

As shown in FIG. 2 (A), a test conduit 27 was connected to the firstoutlet port 15B of the air separator 15, in place of the conduit 23. Atest mirror 25 was held in the air at a distance of about 55 mm awayfrom an end 27E of the test conduit 27. A conical cover 29 was attachednear the end of the test conduit 27 for prevention of the introductionof external air.

In the above-mentioned construction, nitrogen-rich gas was directlyjetted against the test mirror 25. In this test, some oil mist 31 wasobserved to adhere to the surface of the test mirror 25 after about sixhours.

The above-mentioned test indicates that the nitrogen-rich gas stillcontains a small amount of oil mist. Therefore, when the nitrogen-richgas is directly jetted against the bend mirrors 9 to clean the surfacedthereof, although the bend mirrors 9 are not degraded in a short time,there exists a possibility that the bend mirrors 9 deteriorate afterlonger periods of use (e.g., several hours or more).

To overcome the above-mentioned problem, as shown in FIG. 2(B), a filter33 having active carbon (charcoal) was connected midway to the testconduit 27, and a similar test was made. In this case, it was observedthat no oil mist adhered to the surface of the test mirror 25 even after400 hours. The above-mentioned test indicates that it is extremelyeffective to remove oil mist through the use of a filter 33 havingactive carbon.

Accordingly, in the laser beam machine as shown in FIG. 1, it isdesirable to connect an active carbon filter 33 to the conduit 23, inorder to positively remove the small amount of oil mist which stillremains in the nitrogen-rich gas.

It is also possible to interpose the active carbon filter 33 between thefilter 19 and the inlet port 15A of the air separator 15. In this case,however, since the entire volume of compressed air supplied to the airseparator 15 must be filtered, it is preferable to interpose the filter33 on the side of the conduit 23 to increase the useable lifetime of thefilter 33.

The pressure within the optical path cover 11 of the optical system 13is kept higher than the atmospheric (outside air) pressure. It ispossible to construct the optical path cover 11 in such a way that partof the supplied nitrogen-rich gas can be discharged to the outsidethrough an appropriate gap formed between the connection portions ofsome elements of the optical path cover 11. Thus, in this embodiment, inorder to maintain the pressure within the optical path cover 11 under astable constant value, a discharge port 34 is formed at a position ofthe optical path cover 11, and further, a relief valve 35 is connectedto this discharge port 34.

Consequently, the inner pressure within the optical path cover 11 can bemaintained at a substantially constant level through the relief valve35, even if the volume of the optical path cover 11 is decreased orincreased and thereby the inner pressure thereof is increased ordecreased, for example, when the laser beam head 7 is moved. Therefore,it is possible to prevent the bend mirrors 9 from being distorted byfluctuations in inner pressure.

In order to use the nitrogen-rich gas as an assist gas, as well as aprotective gas for the optical path system 13, the discharge port 34 ofthe optical path cover 11 can be connected to the laser beam head 7through a conduit 39 having a shut-off valve 37 connected midwaythereof. In other words, it is possible to use the nitrogen-rich gas inthe optical path cover 11 as an assist gas at need. In this way, thenitrogen-rich gas can be used more effectively.

Further, it is also possible to connect a branch pipe 41 having athrottle valve 43 between the conduit 23 and the laser beam head 7, sothat the nitrogen-rich gas can be directly supplied into the laser beamhead 7 as an assist gas. In this case, as far as the amount of thenitrogen-rich gas within the conduit 23 is sufficient, it is possible touse the nitrogen-rich gas as the assist gas, without exerting a harmfulinfluence upon the pressure within the optical path cover 11.

A second embodiment of the laser beam machine will be describedhereinbelow with reference to FIG. 3. In this embodiment, thenitrogen-rich gas can be supplied to the laser beam head as the assistgas under a stable pressure condition.

In FIG. 3, an air separator 15 is provided to separate oxygen andnitrogen from compressed air. The structure of the air separator 15 isquite the same in structure as with the case of the first embodiment.That is, the air separator 15 is a module in which a great number ofhollow threads (e.g., formed of polyimide) are arranged in a lump in avessel. The hollow polyimide thread membranes are more permeable tooxygen than to nitrogen, both of which are contained in air.

Therefore, when compressed air is supplied into the air separator 15through an inlet port 15A and flows through the hollow threads, oxygen,moisture and oil mist of the compressed air are selectively passedthrough the membranes of the polyimide hollow threads. As a result, itis possible to obtain a nitrogen-rich gas (purity: 94 to 99.5%) from afirst outlet port 15B. On the other hand, the permeated oxygen, moistureand oil mist are discharged through a second outlet port 15C asoxygen-rich gas. The above-mentioned nitrogen-rich gas (obtained byremoving oxygen, moisture and oil mist from air) is dry air (nitrogen)having a dew point of about -50 degrees under atmospheric pressure.

A pressure source 17 such as a compressor is provided to supplycompressed air into the air separator 15. Further, a filter 19 isconnected between the pressure source 17 and the inlet port 15A of theair separator 15 for removal of dust and oil mist contained in thehighly pressurized air provided by the pressure source 17.

In FIG. 3, a first conduit 47 is connected between a first outlet port15B of the air separator 15 and the laser beam head 7 of the laser beammachine. A first pressure control valve 51A, a first pressure gauge 53Aand a first change-over (selector) valve 55A are connected midway of thefirst conduit 47 in sequence. In addition, a muffler (noise eliminator)57A is connected to the first change-over valve 55A.

In order to remotely control the first pressure control valve 51A, apilot path 59 is branched from a connection pipe 45 (connected betweenthe filter 19 and the air separator 15). A first pilot control valve 61Ais connected to the pilot path 59. The first pilot control valve 63A isconnected to the first pressure control valve 51A. Further, a thirdpressure gauge 65A is connected to the pilot path 63A.

A second conduit 49 is connected between a second outlet port 15C of theair separator 15 and the laser beam head 7 of the laser beam machine. Asecond pressure control valve 51B, a second pressure gage 53B and asecond change-over (selector) valve 55B are connected midway of thesecond conduit 49 in sequence. In addition, a muffler (noise eliminator)57B is connected to the second change-over valve 55B.

In order to remotely control the second pressure control valve 51B, asecond pilot control valve 61B is connected to the first pilot controlvalve 61A. A second pilot path 63B is connected between the second pilotcontrol valve 61A and the second pressure control valve 51B. Further, afourth pressure gauge 65B is connected to the second pilot path 63B.

A pressure gauge 67 is connected to the first conduit 47 to display thepressure of the assist gas supplied into the laser beam head 7.

In the above-mentioned construction, when the compressed air of thepressure source 17 is supplied to the air separator 15 through the inletport 15A via the filter 19, since nitrogen and oxygen are separated fromthe supplied compressed air, nitrogen-rich gas (purity: 94 to 99.5%) isdischarged through the first outlet port 15B and the oxygen-rich gas(passed through the polyimide hollow thread membranes) is dischargedthrough the second outlet port 15C of the air separator 15,respectively.

Under these conditions, when the first change-over valve 55A is switchedto an open state and the second change-over valve 55B is switched to aclosed state, the separated nitrogen-rich gas is supplied to the laserbeam head 7 as an assist gas, and the separated oxygen-rich gas isdischarged to the outside through the muffler 57B.

On the other hand, when the first change-over valve 55A is switched to aclosed state and the second change-over valve 55B is switched to an openstate, the separated oxygen-rich gas is supplied to the laser beam head7 as an assist gas, and the separated nitrogen-rich gas is discharged tothe outside through the muffler 57A. Thus, it is possible to selectivelysupply nitrogen-rich gas or oxygen-rich gas into the laser beam head 7as an assist gas according to the laser beam processing conditions.

The pressure of the nitrogen-rich gas supplied to the laser beam head 7can be controlled on the basis of the set pressure of the first pressurecontrol valve 51A, which can be controlled by the first pilot controlvalve 61A. In the same way, the pressure of the oxygen-rich gas suppliedto the laser beam head 7 can be controlled on the basis of the setpressure of the second pressure control valve 51B, which can becontrolled by the second pilot control valve 61B.

For example, when the first pressure control valve 51A is throttled down(the opening is reduced), the volume of the nitrogen-rich gas flowingtherethrough decreases, and the amount of the oxygen-rich gas (passedthrough the hollow thread membranes) increase. Thus, the purity of thenitrogen-rich gas discharged through the first outlet port 15Bincreases. In contrast, when the first pressure control valve 51A isthrottled up (the opening is increased), the amount of gas beingdischarged increases on the side of the first outlet port 15B, and theamount of the oxygen being discharged through the first outlet port 15Bincreases, so that the purity of the nitrogen-rich gas dischargedthrough the first outlet port 15B decreases.

When the second pressure control valve 51B is throttled down oncondition that the first pressure control valve 51A is kept at constantthrottle rate (the set pressure is kept constant), the gasdischargeability decreases on the side of the second outlet port 15C,and therefore the amount of oxygen discharged through the first outletport 15B increases, so that the purity of the nitrogen-rich gasdischarged through the first outlet port 15B decreases.

On the other hand, when the second pressure control valve 51B isthrottled up on condition that the first pressure control valve 51A iskept at a constant throttle rate (the set pressure is kept constant),the gas dischargeability increases on the side of the second outlet port15C, and the amount of the oxygen discharged from the first outlet port15C increases, so that the purity of the nitrogen-rich gas dischargedfrom the first outlet port 15B increases.

As described above, it is possible to keep the purity of thenitrogen-rich gas discharged through the first outlet port 15B of theair separator 15 at any desired value by maintaining a difference inpressure between the first and second conduits 47 and 49 at roughly aconstant value.

During laser beam machining, when the purity of the nitrogen-rich gasobtained through the first outlet port 15B of the air separator 15becomes less than 94%, dress adheres onto cut-off stainless platematerial, for instance. Therefore, in order to cut stainless material,for example, by laser beam cutting processing without producing anydross, it is necessary to keep the purity of the nitrogen-rich gassupplied into the laser beam head 7 at 94% or higher.

In the second embodiment, when the first pilot control valve 61A isoperated to change the set value of the first pressure control valve 51Aso that the pressure of the nitrogen-rich gas supplied to the laser beamhead 7 can be changed according to the machining conditions, adifference in inner pressure between the first conduit 47 and the secondconduit 49 changes.

Further, when the second pilot control valve 61B is operated to changethe set value of the second pressure control valve 51B, it is possibleto set the pressure difference to any desired value, with the resultthat it is possible to keep the purity of the nitrogen-rich gas suppliedfrom the air separator 15 to the laser beam head 7 at a predeterminedvalue without producing dross during the cut-off processing of thestainless material, for instance.

Thus, even if various pressures of the assist gas are required accordingto the laser beam processing conditions, it is possible to suppress thefluctuations in purity of the nitrogen-rich gas supplied to the laserbeam 7, and to control the purity of the nitrogen-rich gas at roughly aconstant level.

Although the first and second pressure control valves 51A and 51B areadjusted, respectively by operating the first and second pilot controlvalves 61A and 61B, separately in this embodiment, it is also possibleto control the first and second pressure control valves 51A and 51B byuse of two electromagnetic pressure reduction valves, respectively.

A third embodiment of the laser beam machine will be describedhereinbelow with reference to FIG. 4. In this embodiment, the first andsecond pressure control valves 51A and 51B are both operated by use of alink mechanism 69, such as a gear or chain link mechanism, for example.When one of the two pressure control valves 51A and 51B is operated, theother of the pressure control valves 51A and 51B is thereby operatedsimultaneously in linkage with the one. In this linkage operation, thefirst and second pressure control valves 51A and 51B are so linked thata difference in inner pressures between the first conduit 47 and thesecond conduit 49 can be always maintained at a substantially constantvalue.

The above-mentioned embodiments have been explained by way of examples.Without being limited only thereto, however, the embodiments can bemodified in various ways. For instance, each of the first and secondpressure control valves can be placed with two flow rate control valveshaving an adjustable throttle, an adjust valve, etc., respectively.Further, the two change-over valves 55A and 55B can be assembled into asingle change-over valve.

Also, when the laser beam machining conditions are roughly constant andtherefore only the nitrogen-rich gas is used as an assist gas, it ispossible to control the pressure of the nitrogen-rich gas supplied tothe laser beam head 7 by use of only the first pressure control valve51A arranged in the first conduit 47. In this case, the second outletport 15C of the air separator 15 is left released to the atmosphericpressure. However, since it is preferable that the purity of thenitrogen-rich gas supplied to the laser beam head 7 is kept at 94% orhigher, the first control valve 51A is controlled in such a way that apressure difference between the first output port 15B and the secondoutlet port 15C can be maintained within a predetermined desired range.

I claim:
 1. A laser beam machine including a laser for generating alaser beam, comprising:a laser beam head; an optical path cover throughwhich said laser beam passes from said laser to said laser beam head; acompressed air supply for supplying compressed air; an air separator forseparating oxygen and nitrogen from said compressed air; first conduitmeans for introducing nitrogen-rich gas separated by said air separatorinto said optical path cover of the laser beam machine as a protectivegas; and second conduit means for introducing the nitrogen-rich gasseparated by said air separator into said laser beam head of the laserbeam machine as an assist gas, said second conduit means including afirst conduit for discharging the nitrogen-rich gas, connected between afirst outlet port of said air separator and said laser beam head, and asecond conduit for discharging the oxygen-rich gas, connected between asecond outlet port of said air separator and said laser beam head; afirst change-over valve disposed midway of said first conduit, forsupplying the nitrogen-rich gas into said laser beam head when opened; asecond change-over valve disposed midway of said second conduit, forsupplying oxygen-rich gas into said laser beam head when opened; a firstpressure control valve disposed midway of said first conduit, forregulating pressure in said first conduit; and a second pressure controlvalve disposed midway of said second conduit, for regulating pressure insaid second conduit, wherein said second conduit means comprises aconduit connected between said optical path cover and said laser beamhead for supplying the nitrogen-rich gas into said laser beam head asthe assist gas.
 2. A laser beam machine including a laser for generatinga laser beam, comprising:a laser beam head; an optical path coverthrough which said laser beam passes from said laser to said laser beamhead; a compressed air supply for supplying compressed air; an airseparator for separating oxygen and nitrogen from said compressed air;first conduit means for introducing nitrogen-rich gas separated by saidair separator into said optical path cover of the laser beam machine asa protective gas; and second conduit means for introducing thenitrogen-rich gas separated by said air separator into said laser beamhead of the laser beam machine as an assist gas, said second conduitmeans including a first conduit for discharging the nitrogen-rich gas,connected between a first outlet port of said air separator and saidlaser beam head, and a second conduit for discharging the oxygen-richgas, connected between a second outlet port of said air separator andsaid laser beam head; a first change-over valve disposed midway of saidfirst conduit, for supplying the nitrogen-rich gas into said laser beamhead when opened; a second change-over valve disposed midway of saidsecond conduit, for supplying oxygen-rich gas into said laser beam headwhen opened; a first pressure control valve disposed midway of saidfirst conduit, for regulating pressure in said first conduit; a secondpressure control valve disposed midway of said second conduit, forregulating pressure in said second conduit; and a control device thatsubstantially constantly maintains the purity of the nitrogen-rich gasby maintaining a substantially constant difference in pressure betweensaid first and second conduits.
 3. The laser beam machine according toclaim 2, wherein said optical path cover further comprises at least onemirror for reflecting said laser beam, and wherein said drynitrogen-rich air is directly jetted against said at least one mirror.4. A laser beam machine including a laser for generating a laser beam,comprising:a laser beam head; an optical path cover through which saidlaser beam passes from said laser to said laser beam head; a compressedair supply for supplying compressed air; an air separator for separatingoxygen and nitrogen from said compressed air; first conduit means forintroducing nitrogen-rich gas separated by said air separator into saidoptical path cover of the laser beam machine as a protective gas; andsecond conduit means for introducing the nitrogen-rich gas separated bysaid air separator into said laser beam head of the laser beam machineas an assist gas, said second conduit means including a first conduitfor discharging the nitrogen-rich gas, connected between a first outletport of said air separator and said laser beam head, and a secondconduit for discharging the oxygen-rich gas, connected between a secondoutlet port of said air separator and said laser beam head; a firstchange-over valve disposed midway of said first conduit, for supplyingthe nitrogen-rich gas into said laser beam head when opened; a secondchange-over valve disposed midway of said second conduit, for supplyingoxygen-rich gas into said laser beam head when opened; a first pressurecontrol valve disposed midway of said first conduit, for regulatingpressure in said first conduit; a second pressure control valve disposedmidway of said second conduit, for regulating pressure in said secondconduit; and a first control valve associated with said compressed airsupply for remotely controlling said first pressure control valve; and asecond control valve associated with said compressed air supply forremotely controlling said second pressure control valve.
 5. A laser beammachine including a laser for generating a laser beam, comprising:alaser beam head; an optical path cover through which said laser beampasses from said laser to said laser beam head; a compressed air supplyfor supplying compressed air; an air separator for separating oxygen andnitrogen from said compressed air; first conduit means for introducingnitrogen-rich gas separated by said air separator into said optical pathcover of the laser beam machine as a protective gas; and second conduitmeans for introducing the nitrogen-rich gas separated by said airseparator into said laser beam head of the laser beam machine as anassist gas, said second conduit means including a first conduit fordischarging the nitrogen-rich gas, connected between a first outlet portof said air separator and said laser beam head, and a second conduit fordischarging the oxygen-rich gas, connected between a second outlet portof said air separator and said laser beam head; a first change-overvalve disposed midway of said first conduit, for supplying thenitrogen-rich gas into said laser beam head when opened; a secondchange-over valve disposed midway of said second conduit, for supplyingoxygen-rich gas into said laser beam head when opened; a first pressurecontrol valve disposed midway of said first conduit, for regulatingpressure in said first conduit; a second pressure control valve disposedmidway of said second conduit, for regulating pressure in said secondconduit; and a link mechanism for controlling said first and secondpressure control valves simultaneously in linkage, to maintain asubstantially constant difference in pressure between said first andsecond conduits, said link mechanism linking said first and secondpressure control valves.
 6. A laser beam machine including a laser forgenerating a laser beam, comprising:a laser beam head; an optical pathcover through which said laser beam passes from said laser to said laserbeam head; an air separator for separating oxygen and nitrogen fromcompressed air; first conduit means for introducing nitrogen-rich gasseparated by said air separator into said optical path cover of thelaser beam machine as a protective gas; and second conduit means forintroducing the nitrogen-rich gas separated by said air separator intosaid laser beam head of the laser beam machine as an assist gas, saidsecond conduit means comprising a conduit connected between said opticalpath cover and said laser beam head for supplying the nitrogen-rich gasinto said laser beam head as the assist gas.
 7. A laser beam machineincluding a laser for generating a laser beam, comprising:a laser beamhead; an optical path cover through which said laser beam passes fromsaid laser to said laser beam head; an air separator for separatingoxygen-rich gas and nitrogen-rich gas from compressed air; conduit meansfor introducing said nitrogen-rich gas separated by said air separatorinto said laser beam head of the laser beam machine as an assist gas,said conduit means comprising: a first conduit connected between a firstoutlet port of said air separator and said laser beam head, fordischarging the nitrogen-rich gas; a first change-over valve disposedmidway of said first conduit, for supplying the nitrogen-rich gas intosaid laser beam head when opened; a first pressure control valvedisposed midway of said first conduit, for controlling pressure in saidfirst conduit; a second conduit connected between a second outlet portof said air separator and said laser beam head, for discharging saidoxygen-rich gas; a second change-over valve disposed midway of saidsecond conduit, for supplying oxygen-rich gas into said laser beam headwhen opened; a second pressure control valve disposed midway of saidsecond conduit, for controlling pressure in said second conduit; and acontrol device that substantially constantly maintains the purity of thenitrogen-rich gas by maintaining a substantially constant difference inpressure between said first and second conduits.
 8. The laser beammachine according to claim 7, wherein said optical path cover furthercomprises at least one mirror for reflecting said laser beam, andwherein said dry nitrogen-rich air is directly jetted against said atleast one mirror.